Tectonic Puzzle: Why West Africa Didn't Follow South America

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South America nearly carried off Northwest Africa when the
world's last supercontinent fell apart 130 million years ago.
Now, a new model helps explain why the Sahara settled east of the
Atlantic instead of sailing off with South America — it's all
about the angles.

Back before the Atlantic Ocean formed, Africa and South America
nestled together in a massive supercontinent called Gondwana.
When this landmass started to split, gashes in Earth's crust
called rifts opened up along pre-existing weaknesses.

One of these gashes, called the West Africa Rift System, started
to tear apart the future Sahara desert. Two more rifts formed
along the future boundaries of South America and Africa. Imagine
three rift
zones, two lined up essentially north-south and one pointing
east-west. These alignments are key to explaining why the
continents broke apart the way they did, according to a study
published March 6 in the journal Geology.

The planet's plate
tectonic forces could more easily pull apart the two
continents at the east-west–oriented rift than at the
north-south–oriented rift in the Sahara desert, the researchers
found.

"The direction in which the continents break apart heavily
influences the success of the rift system," said study co-author
Sascha Brune, a geophysicist at GFZ Potsdam in Germany. "Because
the rift system was at a very low angle to the extension
direction, this rift won out in the end," he told Live Science's
Our Amazing Planet.

At that time, South America was heading westward. "Plates are
pulled apart by large-scale geological forces that come from the
plate boundary or the mantle, but for the rift, it's not
important where these forces come from," Brune said. "If you pull
more in the direction of the rift, you need two times less force
to get the rift going." The mantle is the hotter layer of rock
beneath Earth's crust.

The crust often breaks apart at three-pronged junctions, such as
the triple rift that formed between Africa-South America, and
it's not uncommon for one rift to fail to develop. The model
developed by Brune and his co-authors suggests that the angle
between the rift and the plate tectonic forces plays an important
role in determining which rifts will fail.